The present invention relates generally to scroll-type machinery. More particularly, the present invention relates to scroll-type machinery specifically adapted for use in the compression of fuel gas and the control system for the scroll-type machinery.
Scroll machines are becoming more and more popular for use as compressors in refrigeration systems as well as air conditioning and heat pump applications due primarily to their capability for extremely efficient operation. Generally, these machines incorporate a pair of intermeshed spiral wraps, one of which is caused to orbit with respect to the other so as to define one or more moving chambers which progressively decrease in size as they travel from an outer suction port towards a center discharge port. An electric motor is normally provided which operates to drive the scroll members via a suitable drive shaft.
As the popularity of scroll machines increase, the developers of these scroll machines continue to adapt and redesign the scroll machines for compression systems outside the traditional refrigeration systems. Additional applications for scroll machines include helium compression for cryogenic applications, air compressors, fuel gas compressors for distributed power generation and the like. The present invention is directed towards a scroll machine which has been designed specifically for the compression of fuel gas and the control system which operates the compressor in order to supply compressed fuel gas for distributed power generation.
Distributed power generation has emerged in recent years as a means to provide on-site power generation for commercial and industrial customers seeking a degree of independence from the possibility of a power shortage or power loss. While previous distributed power generation equipment was designed primarily to address the need for backup power, today""s products are focused on providing continuous reliable power at an attractive price. Specifically, today""s distributed power generators are intended to continuously supply clean, quiet and reliable power for both grid parallel and stand alone applications.
One important vehicle for the emerging distributed power generation market is the microturbine power generators. This device, about the size of two refrigerators, contains a jet turbine engine capable of using multiple fuels including pressurized fuel gas. Inlet air is compressed in the centrifugal compressor section, mixed with pressurized fuel gas, and then combusted to drive a turbine and a generator on a common high-speed shaft with the compressor. The high frequency power is then rectified and converted to a useable 50/60-cycle three-phase power through the use of an onboard inverter. Single microturbine generators are currently sized for 30 to 100 kilowatts of power generation but may eventually service a 200 to 300 kilowatt load. Fuel sources for microturbines include pipeline quality natural gas and biogas from landfill and digester plants.
Another technology well suited for distributed power generation is a conventional diesel driven generator converted for use with pressurized fuel gas. In this application termed xe2x80x9cdual fuelxe2x80x9d, a small percentage of diesel fuel is mixed with pressurized fuel gas to enhance the power generation output of the reciprocating engine. Low emissions are obtained relative to conventional diesel gensets, allowing this equipment to be used for continuous power generation versus the limited use operation allowed previously with emergency power applications. Dual fuel diesel gensets are being developed for power needs up to several megawatts.
An additional potential application option for the fuel gas compressor is a fuel cell using natural gas as the fuel. With this device pressurized natural gas flows through a reformer element which separates out hydrogen from the methane in the natural gas. The hydrogen fuel is then combined with pressurized air (oxygen) to provide the necessary ingredients for the electrochemical reaction that results in DC electric power.
To meet the need of these emerging power generation technologies for pressurized fuel gas, a reliable and efficient gas compression system was required to boost gas pressure at the site to the typical 60-100 psig operating pressure needed by the equipment. Normal variability in gas pressure and energy content, as well as the need for the power generator to operate at part load, required this gas compression system to efficiently supply a variable amount of fuel. This requirement is accomplished by the present invention through a custom variable speed electronic drive that also includes a microcompressor based logic control for use in fault and safety mode detection. Finally, to insure many years of reliable operation, a proven compressor technology, utilized in air conditioning and refrigeration products, was adapted to meet the specific needs of fuel gas compression.
The cyclic compression of fuel gas presents very unique problems with respect to compressor design because of the high temperatures encountered during the compression process. The temperature rise of fuel gas during the compression process can be more than twice the temperature rise encountered during the compression process of a conventional refrigerant. In order to prevent possible damage to the scroll machine from these high temperatures, it is necessary to provide additional cooling for the scroll machine in addition, fuel gas compression systems as well as other compression applications need to be capable of being powered from a variety of electrical sources. These electrical sources can be a direct current source or an alternating current source depending upon the particular application.
The present invention, in one embodiment, comprises a scroll compressor system which is specifically adapted for use in the compression of fuel gas. The scroll compressor of the system includes the conventional low pressure oil sump in the suction pressure zone of the compressor as well as a second high pressure oil sump located in the discharge pressure zone. An internal oil cooler is located within the low pressure oil sump. Oil from the low pressure oil sump is circulated to the bearings and other movable components of the compressor in a manner similar to that of conventional scroll compressors. A portion of the oil used to lubricate these moving components is pumped by a rotating component onto the windings of the electric motor to aid in cooling the motor. The oil in the high pressure oil sump is routed through an external heat exchanger for cooling and then is routed through the internal oil cooler located in the low pressure oil sump. From the internal oil cooler, the oil is injected into the compression pockets to aid in the cooling of the compressor as well as to assist in the sealing and lubrication of the intermeshed scroll wraps. An internal oil separator is provided in the discharge chamber to remove at least a portion of the injected oil from the compressed gas and thus replenish the high pressure oil sump. An oil overflow orifice prevents excessive accumulation of oil in the high pressure oil sump. A second external oil separator is associated with the external heat exchanger in order to remove additional oil from the natural gas to provide as close as possible for an oil free pressurized natural gas supply.
In another embodiment of the present invention, a unique scroll type compressor which is modified from proven air conditioning scroll compressor technology is provided for compressing the fuel gas. The compressor is a hermetic design which means both the motor and the scroll compression mechanism are in the same enclosure. This eliminates shaft seals and the possibility of gas leakage as is possible with open drive type compressors. Due to the high specific heat ratio and high compression temperatures inherent with fuel gas, the compression process is oil flooded to prevent overheating and insure compressor durability. Compressor durability is also enhanced by the lower outlet pressures of this application relative to the higher pressures typical in air conditioning applications. Both UL and CE approval have been obtained for this product.
The control system of the present invention allows the powering of the compressors by either a direct current (DC) source or an alternating current (AC) source. The system can be designed to be powered by only a DC source, only an AC source or it can be a xe2x80x9cuniversalxe2x80x9d compressor which can be powered by either a DC or an AC source.
Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.